Possible role of protein modification system (PMS) which modifies the existing proteins at the NH2-terminal end with arginine generated from arginyl tRNA by arginyl tRNA transferase will be studied in relation to cellular aging. Comparison of the PMS of human fibroblasts (IMR 90) at various cellular passages as well as in rat tissues of different ages will be carried out by studying 1) electrophoretic analysis of products before and after modification by PMS; 2) product analysis of drug treated chromatin (such as hydrocortisone, aspirin, prostaglandins) before and after modification by PMS; 3) analysis of isoaccepting species of tRNA isolated from various ages; 4) transcriptional capacity of chromatin with mammalian RNA polymerase before and after modification by PMS; 5) effect of cellular transformation or infection of late passaged IMR 90 by SV40 on PMS activity.